Moon Phase Calculator
Find the Moon's phase on any date — its age in days, the illuminated fraction, and the dates of the next new and full moons — from the mean lunar cycle.
How to use this calculator
- 1Enter the date you want the Moon's phase for.
- 2Read the phase name and how much of the Moon is illuminated.
- 3Check whether it's waxing (growing) or waning (shrinking).
- 4See the dates of the next full and new moons.
How it works
Moon phase
moon age = (days since a known new moon) mod 29.53 phase fraction = age ÷ 29.53 illuminated fraction = (1 − cos(2π · phase fraction)) ÷ 2 synodic month = 29.53 days (new moon to new moon)
The Moon's phase depends on where it is in its cycle relative to the Sun as seen from Earth, and that cycle — the synodic month — averages 29.53 days from one new moon to the next. Counting the days elapsed since a known new moon and taking the remainder after dividing by 29.53 gives the Moon's 'age': 0 at new moon, growing to about 14.75 at full moon, and returning toward 29.53. The illuminated fraction we see is not proportional to the age but follows a cosine curve, because it depends on the angle between the Sun, Moon, and Earth — the lit portion grows slowly near new and full and fastest around the quarters. The eight named phases divide the cycle into equal segments: new, the waxing crescent, first quarter, waxing gibbous, full, then waning back through gibbous, last quarter, and crescent.
Worked example
Counting the days from a reference new moon on 6 January 2000 and taking the remainder over 29.53 days gives the Moon's age for any date. An age near 0 is a new moon (dark), near 7.4 a first quarter (half lit and waxing), near 14.8 a full moon (fully lit), and near 22 a last quarter.
Moon Phase Calculator: the complete guide
What causes the phases
The Moon does not produce its own light; it shines by reflecting sunlight, and exactly half of it is always lit — the half facing the Sun. The phases arise entirely from our viewing angle. As the Moon orbits the Earth over roughly a month, the angle between the Sun, the Moon, and us changes, so we see different portions of that permanently-lit half. When the Moon is between Earth and the Sun, its lit side faces away from us and we see a dark new moon; when Earth sits between the Sun and Moon, the lit side faces us fully and we see a full moon.
This geometry explains why the phases are a smooth cycle rather than a set of discrete states. Between new and full the Moon waxes, showing an ever-larger sunlit slice — crescent, then quarter, then gibbous. After full it wanes back through the same shapes in reverse. The 'quarter' names, which confuse many people because the Moon looks half-lit, refer to the Moon being a quarter and three-quarters of the way through its cycle, not to the fraction illuminated.
The synodic month and why it's longer than the orbit
The cycle of phases takes 29.53 days, called the synodic month, but the Moon actually completes an orbit around the Earth in only about 27.32 days — the sidereal month. The difference is not an error; it comes from the Earth's own motion around the Sun. In the time the Moon takes to orbit once, the Earth has moved along its own orbit, so the Moon must travel a little farther to return to the same position relative to the Sun and restore the same phase. Those extra two-plus days are the gap between the orbital period and the phase cycle.
This distinction matters for prediction. The phases follow the synodic month, so that is the number to use when calculating when the next full moon falls. The 29.53-day figure is an average, because the Moon's orbit is elliptical and its speed varies, making individual cycles a little longer or shorter. A calculation based on the mean value, like this one, tracks the phases to within roughly a day over long spans — fine for knowing whether tonight's moon is waxing gibbous, but not for the minute-precise predictions that eclipses and tides require.
Phases in culture, nature, and the calendar
The lunar cycle is one of humanity's oldest clocks, and its fingerprints are everywhere. The very word 'month' derives from 'moon', and many calendars — the Islamic, Hebrew, and traditional Chinese among them — are lunar or lunisolar, timing their months and festivals to the phases. The new moon marks the start of the month in these systems, and holidays like Ramadan, Passover, and the Chinese New Year are set by the Moon rather than the Sun, which is why their dates drift through the solar year.
The phases also govern the tides, through the Moon's gravity, with the largest spring tides at new and full moon when the Sun and Moon pull in line, and the gentlest neap tides at the quarters. In the natural world, many species time spawning, migration, and activity to the lunar cycle and its moonlight. And for the amateur astronomer, the phase is a practical planning tool: the days around new moon, with dark skies, are best for observing faint galaxies and nebulae, while the bright full moon washes them out but beautifully lights the lunar surface itself. Knowing the phase, which this calculator supplies for any date, is the first step in planning a night under the stars.
Frequently asked questions
How is the moon phase calculated?
By counting the days since a known new moon and taking the remainder over the 29.53-day lunar cycle, which gives the Moon's age. The age maps to a phase — 0 is new, about 14.8 is full — and the illuminated fraction follows a cosine of the age's position in the cycle.
Why is the lunar cycle 29.5 days but the orbit 27.3?
The Moon orbits Earth in 27.32 days (the sidereal month), but the phases take 29.53 days (the synodic month). The difference is because Earth moves around the Sun meanwhile, so the Moon must travel a bit farther to return to the same Sun-Earth-Moon angle that defines a phase.
What does waxing and waning mean?
Waxing means the illuminated part is growing, from new moon toward full; waning means it's shrinking, from full back toward new. A waxing crescent appears after the new moon and a waning crescent before the next one. The Moon is waxing for the first half of the cycle and waning for the second.
How accurate is this moon phase calculation?
To about a day, since it uses the mean 29.53-day cycle rather than the Moon's slightly irregular real orbit. That's enough to name the phase and estimate illumination for any date, but precise timing of eclipses, tides, or moonrise needs a more detailed model of the actual orbit.